Troubleshooting MICROMASTER 420 STARTER Parameter Change Errors

The Siemens MICROMASTER 420 (6SE6420 series) is a compact, sensorless vector (SLVC) drive widely used in constant-torque and variable-torque applications. Commissioning is performed with the STARTER (or newer Startdrive) software, which stores a complete parameter image both online (in the drive) and offline (in the project file on the engineering PC). When STARTER refuses to write a parameter and reports an "error during change parameter" or "parameter cannot be changed in this state" dialog, the fault is most often attributed to a corrupted offline parameter file rather than a drive hardware defect. This reference walks through the diagnostic path, the field-proven resolution used by commissioning engineers, and the preventive measures that keep a fleet of MICROMASTER 420 drives synchronized.

1. Problem Definition

1.1 Symptom Profile

Engineers report one or more of the following dialogs in STARTER when trying to change a writable parameter (P0000–P1999 read-only, P2000–P3999 writable) on a MICROMASTER 420:

• "Error during change parameter" dialog box.
• "Parameter write protected" or "Parameter cannot be changed in current operating state."
• "Value out of range" rejection even for an unchanged default value.
• RAM-to-EEPROM commit fails; the value reverts after power-cycle.
• "Download to target device failed" when a RAM/ROM copy is attempted.

1.2 Trigger Conditions

1.3 Affected Versions

• Firmware: MICROMASTER 420 firmware V1.0 through V3.2 (the last released branch on this platform).
• STARTER: V4.1.1, V4.2, V4.3, V4.4, V4.5, V5.0, V5.1, V5.3, V5.4 SP3–SP5. Errors are reported most often in V4.x where the offline file format is closer to legacy DOT projects.
• Communication: USS on RS485 (default), PROFIBUS DP on a CB15/CB20 module, or MODBUS RTU on the integrated terminal.

2. Root Cause Analysis

2.1 Primary Cause: Corrupted Offline Parameter File

STARTER maintains two parameter images:

1. Online image – read from the drive over the active bus. This is the ground truth.
2. Offline image – stored in the project directory as *.pnx, *.dproj, or *.zip archives. This is what STARTER uses to compare and to issue writes.

When the offline image becomes corrupted – typically through an interrupted upload, a forced Windows shutdown during a Save All, or a copy-paste of a project tree – the checksum inside the offline file no longer matches the parameter set that STARTER expects. STARTER then refuses to commit writes because the compare-and-validate step at the heart of the change-parameter dialog fails. The drive itself is healthy; only the engineering-side file is invalid.

2.2 Secondary Causes

2.3 Why Two Identical Drives Behave Differently

The first drive (address 31) accepts the original project because the offline-to-online compare succeeds. The second drive (address 32) triggers the error because STARTER's diff engine now detects that certain internal pointer fields (e.g., offline_checksum and topology_uuid) do not match the file's content. The fault is in the file, not in the drive; therefore, downloading parameters from the working drive (31) into the problem drive (32) by first re-addressing drive 31 to 32, performing an Upload from Target Device, then writing the result back to drive 32, repairs the offline image and unblocks parameter changes.

3. MICROMASTER 420 Parameter Architecture

3.1 Parameter Numbering Scheme

3.2 Indexed Parameters and BICO Sources

Many MM420 parameters are index-dependent (e.g., P2011[0] = RS485 address, P2011[1] = BOP link address). Failure to specify the index when writing from STARTER can return a generic "parameter change error." Use the indexing drop-down in the parameter view rather than the simple input field.

4. STARTER Software Parameter File Layout

A STARTER project for a MICROMASTER 420 contains the following artifacts:

Whenever STARTER writes a parameter, it performs the following steps internally:

1. Read the parameter from the offline file.
2. Validate against the schema for the firmware version (read from r0018 / r0019 firmware version).
3. Compute the cyclic redundancy check (CRC) of the parameter set.
4. Compare offline vs. online value (if Compare Values is enabled).
5. Issue a parameter write via the active bus (PKW/PZD for PROFIBUS, USS frame for RS485).
6. Read back and commit to EEPROM (P0971 = 1 trigger).

If step 2 or step 3 fails – typically because the offline CRC was invalidated by a partial save – STARTER raises the "error during change parameter" dialog and aborts the write.

5. Diagnostic Procedure

5.1 Step 1 – Confirm Drive Online State

1. Right-click the drive in the project tree, choose Online / Offline.
2. Observe the symbol next to the drive. A green tick = online, yellow = warning, red X = fault or no communication.
3. If the symbol is red, check RS485 polarity on terminals 29 (+) and 30 (-–).

5.2 Step 2 – Read the Drive’s Firmware Version

From the parameter list, read:

• r0018 – Firmware version (e.g., 1.05)
• r0019 – Drive object ID
• r0021 – Actual DC-link voltage (must be > 430 V for 400 V class units, > 215 V for 230 V class units)
• r0026 – Actual DC-link current
• r0034 – Motor thermal load (%)

5.3 Step 3 – Read the Active Fault Buffer

Faults and warnings are stored as P0947 (last fault code) and P0952 (count). Common MM420 fault codes that block parameter writes:

5.4 Step 4 – Check P0927 (Write Protection) and P0010 (Commissioning Filter)

• P0927 = 0 – writes allowed.
• P0927 = 1 – writes blocked. Reset to 0 to enable parameter modification.
• P0010 = 0 – ready; most parameters writable.
• P0010 = 1 – quick commissioning; only motor nameplate parameters writable.
• P0010 = 30 – factory reset; do not enter for parameter write troubleshooting.

5.5 Step 5 – Inspect Offline Project CRC

From the STARTER menu, choose Project → Diagnostics → Check Project Integrity. STARTER reports a CRC mismatch for the offline parameter file. The dialog is the definitive confirmation that the project file is corrupted.

6. Resolution Methods

6.1 Method A – Refresh Offline Image by Re-uploading from the Drive (Field-Proven)

This is the procedure that resolved the reported case where two drives shared the same corrupted offline file.

1. On the working drive (USS address 31), change P2011[0] to 32. Press the green checkmark on the BOP to commit. The drive is now reachable at the problem address.
2. In STARTER, change the offline USS address of the project to 32 and click Online → Connect to Target System.
3. Once online, right-click the drive and choose Upload from Target Device (HW). STARTER rebuilds the offline parameter image directly from the live drive.
4. Compare the new offline file with the original by choosing Online → Compare Values. All parameters should show a green match.
5. Open any writable parameter (e.g., P1120 ramp-up time) and change the value. Click OK. The write should now succeed without error.
6. Restore the working drive’s address (P2011[0] = 31) to put the network back into its original topology.

6.2 Method B – Repair the Offline File In-Place

1. Make a backup of the corrupted .pnx / .dproj file.
2. Open STARTER. File → Open Project with the corrupted file.
3. Right-click the drive and choose Expert List → Reload from Device.
4. When the reload completes, Save Project with a new name to overwrite the corrupt CRC.

6.3 Method C – Factory Reset and Re-Commissioning

1. Set P0010 = 30 and P0970 = 1. The drive resets to factory defaults (takes approximately 10 s).
2. Cycle the 24 V control power or the line supply to ensure the EEPROM commit is finalized.
3. Re-enter the motor data from the motor nameplate: P0304 (V), P0305 (A), P0307 (kW or HP), P0310 (Hz), P0311 (rpm).
4. Run motor data identification: P1900 = 2 (full identification) and issue an ON command. Wait for the drive to complete the auto-tune (typically 30–60 s without load).
5. Set application parameters: P0700 = 1 (terminals), P1000 = 2 (analog setpoint), P1300 = 20 (SLVC), P1120 = 10 s, P1121 = 10 s, P2010 = 6 (9600 baud USS), P2011 = 31 (USS address).

6.4 Method D – STARTER Reinstallation

1. Use Control Panel → Programs and Features to uninstall STARTER.
2. Delete the residual folders: C:\Program Files\Siemens\Automation\STARTER and C:\Users\<user>\AppData\Roaming\Siemens\STARTER.
3. Reinstall STARTER from the official Siemens support package. The latest version supporting MICROMASTER 420 is available as STARTER V5.4 SP5 with the HSP for legacy MM4 drives installed.
4. Restore the project file and repeat Method A to refresh the offline image.

7. Verification Procedure

After applying any of the four methods, verify the repair as follows:

1. Write test: Change P1120 ramp-up time from its current value to 11.0 s and back. Both writes should complete without error.
2. Compare test: In STARTER, choose Online → Compare Values. The summary at the bottom should report 0 differences if you have not actually changed anything.
3. Power-cycle test: Power down the drive for at least 30 s. Power up. Re-read the parameter that was changed. It should retain the new value, confirming the EEPROM commit succeeded.
4. Fault buffer test: Read r0947[0..7] and r2133[0..7]. Neither should contain F0051 (parameter fault) following the test writes.
5. Communication test: From a host PLC, perform a read of P2011[0] (USS address) and verify that the response contains the expected value within one polling cycle.

8. Prevention and Best Practices

8.1 Project Hygiene

• Never clone a STARTER project by Windows copy-paste. Use Save As to generate a new project with a fresh internal UUID.
• Always perform Save Project from the STARTER menu – do not rely on auto-save, which can leave intermediate CRCs on disk.
• Enable Automatic Backup of Parameter Sets in Options → Settings → Commissioning so that a daily snapshot of every project’s offline file is kept under %LOCALAPPDATA%\Siemens\STARTER\Backup.
• Maintain a versioning convention: append the firmware version and date, e.g., LineA_MM420_V105_20240610.pnx.

8.2 Communication Hygiene

• Use a unique USS address for every drive. Common address allocations: 1–Master, 11–20 – Line 1, 21–30 – Line 2, 31–40 – Pumps, 41–50 – Conveyors.
• Terminate the RS485 bus only at the two physical ends with 120 Ω resistors, not on every node.
• Set P2014[0] USS telegram off-time to a value larger than the slowest PLC scan (typically 1000 ms).
• Run a CRC check on the physical layer by reading r0052 status bit 13 (USS error counter) after each modification cycle. A value > 0 indicates bus noise.

8.3 Commissioning Workflow

1. After any parameter change, click Copy RAM to ROM (or set P0971 = 1) so the value is committed to non-volatile memory.
2. Cycle the drive offline / online and verify the parameter persists.
3. Export the final parameter set to a PDF for records retention: Project → Print → Print to PDF.
4. Annotate any non-default parameters with a comment in the offline project; the comment is stored in the project file and survives firmware changes.

9. Related Error Scenarios

10. Reference Material

• Siemens MICROMASTER 420 product support page – firmware downloads, manuals, FAQs.
• STARTER commissioning tool download and release notes – latest SP that retains the legacy MM4 support package.
• MICROMASTER 420 operating instructions (PDF) – complete parameter list, fault codes, and BICO diagrams.